1. Field of the Invention
The present invention relates to an image-recorded medium on which images capable of visual recognition by both reflection light and transmission light are recorded, particularly to an image forming method and image-recorded medium that provide highly reliable recorded images which do not show changes with time under various environments.
2. Description of the Related Art
With recent developments of image forming technologies, methods for cheaply forming a large quantity of images having the same quality by employing various printing methods such as intaglio printing, letter-press printing, planographic printing, gravure printing and screen printing, have been known in the art.
For example, automobile meter panels are required to have both characteristics of light screening property and light transmitting property, and the screen printing has been employed from the technical point of view.
Recently, production of the automobile meter panels by applying electrophotographic technologies, has been attempted. By applying the electrophotographic technologies, the preparation and management of screen meshes, which have been required in screen printing, becomes unnecessary; moreover, minor changes of the meter panel can be easily made by changing electronic information, which largely reduces production cost in small lot production; further, repetition of processes such as printing and drying becomes unnecessary, which largely improves productivity.
With respect to laminates obtained by laminating recorded members such as identification cards and other cards with a transparent resin film, electrophotographic copy machines that produce and laminate the recorded medium by the electrophotographic process are proposed (for example Japanese Patent Application Laid-Open (JP-A) No. 9-171278)).
In general, recorded images having both characteristics of screening and transmitting lights are required to have a proportion of acceptable products of almost 100% from the view point of mass production. However, complete light screening property cannot be attained by direct application of conventional electrophotographic technologies: many light transmitting holes called pin-holes are generated when electrophotographic technologies are employed. Accordingly, such high proportion of acceptable products cannot be obtained by conventional electrophotography.
For solving these problems, the following two countermeasures are essential for applying the electrophotographic process:
(1) when images are recorded by laminating four toner layers, at least two of the layers are composed of black toners; and
(2) the transferred mass per area (TMA) of toner at light screening portions are at least 2.1 mg/cm2.
A light screening ratio having a transmission density of 3.5 or more may be achieved by the two countermeasures. However, if the countermeasures are employed in usual electrophotographic systems, sufficient image quality cannot be obtained: the TMA twice as high as that of conventional systems causes problems such as image breaking caused by transfer failures or blisters (foaming in the fixing process). In other words, these transfer failures and blisters cause defects called pin-holes when the recorded image is evaluated with the transmission light.
In order to prevent the generation of pin-holes, measures are employed. For example, the secondary transfer bias is optimized for avoiding transfer failure, or only the toner on the uppermost surface of the toner layer is temporary fixed on a medium to an extent not to cause image slippage then secondarily fixed in an oven in a non-contact manner for preventing blisters.
For example, meter panels equipped in an automobile are required to have long-term reliability in an environment where the temperature in a cabin in midsummer is supposed to be 80 to 100° C. Since this temperature is higher than the glass transition point of the toner, the toner is fluidized by heat, so that the recorded images can no longer kept intact.
Thus, lamination after the second fixing is essential for protecting the recorded image. Only panel portions are cut from the meter panel precursor produced as described above by a cutout device and attached to a panel frame to produce a meter panel.
However, when the toner layer is not sufficiently degassed during the secondary fixing in an oven, the air in toner layers is expanded to put away the softened toner. The portions of the toner are grown into voids that transmit the light to deteriorate the reliability of the recorded images.